EVAPORATION IN THE SURFACE ENERGY BALANCE 21 



Lastly, the definitions of the three coefficients of eddy transport are 

 given: 



eddy viscosity, Km = — ^ (4) 



"i 



where, for practical purposes, t (the momentum flux) can be considered 

 effectively constant = tq 



E 



eddy diffusivity, Kw = — ^r- (5) 



dz 



where x is absolute humidity and E is the flux of vapour, also effectively 

 equal to Eq the evaporation at the surface. 



eddy conductivitv, Kh= — -j^ T (^) 



where P is the dry adiabatic lapse rate and Q is the flux of sensible heat. 

 The eddy transfer of momentum, water vapour and sensible heat involves 

 three aspects of atmospheric diffusion. Considerable progress in the subject 

 has been made by assuming the equahty of at least two of the diffusivities, 

 and sometimes of all three. Thus there is a distinction from the somewhat 

 analogous case of molecular diffusion, from which the mixing length 

 concept originally arose. Two main compHcations should be kept in mind. 

 For Ri ^ 0, eddy viscosity becomes comparable with molecular diffusion 

 as fully turbulent flow degenerates into laminar flow. For Ri <^ 0, free 

 convection affects the transport mechanism and might be expected to 

 introduce (or increase) differences between the diffusivities. Between these 

 extremes, it can be assumed to a first approximation that the diffusivities 

 are equal. 



3. ENERGY BALANCE CONSIDERATIONS 



With sufficient accuracy for nearly aU practical purposes, the energy balance 

 at the earth's surface may be expressed by the equation : 



R + XE+S+Q=o (7) 



where JR. is the net flux of short- and long-wave radiation, S is the flux of 

 heat in the ground and A is the latent heat of evaporation. 



In the last few years it has been possible to place more and more confi- 



